Richard J. Briggs

Measurements of hose instability of a relativistic electron beam

The observed disruption of a self‐focused, relativistic electron beam propagating through a gas is shown to result from the growth of m=1 (’’kink’’ or ’’hose’’) perturbations. Measurements of the frequency dependence of the spatial amplification rate are presented. An upper cutoff to the frequency range for hose amplification is observed, in agreement with a theoretical model that includes the damping effects of a spread in the particle betatron frequency.

Plasma current and conductivity effects on hose instability

Hose instability dispersion relations, which include a self‐consistent treatment of the spatial and temporal evolution of plasma conductivity and plasma current, are derived for a relativistic beam propagating in weakly ionized gas. A simplified conductivity model is used which neglects temperature dependence of the electron mobility. In some regimes the results are dramatically different from those found previously for a beam propagating in a fixed conductivity channel. For example, the hose growth rate is found to decrease with increasing current Ib for a beam propagating in initially neutral gas, even though the plasma return current fraction increases rapidly with Ib. As another example, it is found that an externally driven discharge current can completely eliminate hose instability in a fixed conductivity channel, but causes only a weak decrease in growth rate when the plasma conductivity is modeled self‐consistently. OFF

Space‐charge waves on a relativistic, unneutralized electron beam and collective ion acceleration

The space‐charge‐wave dispersion characteristics of a thin, hollow, relativistic electron beam is calculated for beam currents near the space‐charge limit for propagation of the beam.

Radial expansion of self-focused, relativistic electron beams

Measurements of the radial expansion from gas scattering of a low ν/γ relativistic electron beam are presented. The measured current density profiles approach a Bennett shape, as predicted theoretically, and the rate of expansion of the beam radius with distance is in good agreement with the predictions of an envelope equation. locus has been determined. It terminates at zero temperature at

Experiments on Surface Wave Propagation along Annular Plasma Columns

The propagation of surface waves along a plasma column of annular cross section was investigated experimentally. The laboratory plasma used for the experiment was a mercury‐vapor dc discharge. The properties of the experimental discharge tube were examined to show some basic differences between the laboratory plasma and the idealized model of the plasma used for the analysis. In particular, a radially non‐uniform density distribution and the variation of the distribution with the applied magnetic field was noted. In spite of the radial‐density inhomogeneity, the experimentally determined phase constants of the backward‐surface wave are in good agreement with values predicted from the uniform density theory. The attenuation in the absence of the magnetic field is consistent with collisional losses, predominantly with the walls of the container. The effect of an axial magnetic field on the surface‐wave characteristics is examined. Experimental results show that in the presence of a magnetic field the attenu...

Efficient trapping of high‐level E layers in a strong toroidal field

Efficient trapping of high‐level E layers in the presence of a strong toroidal field (Bθ∼ Bz) has been achieved in the astron experiment at Lawrence Livermore Laboratory. The beam was injected into hydrogen gas at pressures around 0.1 mTorr resulting in the prompt formation of plasma with a density of 1012/cm3, Single‐pulse trapping efficiencies greater than 50%, circulating currents in excess of 13 kA with a typical half‐life of 1 msec, and diamagnetic strengths (ζ) up to 75% were achieved. Using the multiple pulse capability of the astron linac, “maintenance” of an E layer at ζ ∼ 30% was demonstrated.

High Voltage Operation of Helical Pulseline Structures for Ion Acceleration

To accelerate ions using a helical pulseline requires the launching of a high voltage traveling wave with a waveform determined by the beam transport physics in order to maintain stability and acceleration. This waveform is applied to the front of the helix, creating a steep voltage ramp that moves down the helix, accelerating ions over distances much longer than the ramp length. An oil dielectric helix to demonstrate ion acceleration has been designed and fabricated. Helix design parameters, high voltage issues, input coupling methods, termination methods, and pulsers are described. Waveforms from the initial characterization of the oil dielectric helix are also described.

Applications of Ion Induction Accelerators

As discussed in Chap. 9, the physics of ion induction accelerators has many commonalities with the physics of electron induction accelerators. However, there are important differences, arising because of the different missions of ion machines relative to electron machines and also because the velocity of the ions is usually non-relativistic in these applications. The basic architectures and layout reflects these differences. In Chaps. 6, 7, and 8 a number of examples of electron accelerators and their applications were given, including machines that have already been constructed. In this chapter, we give several examples of potential uses for ion induction accelerators. Although, as of this writing, none of these applications have come to fruition, in the case of heavy ion fusion (HIF) , small scale experiments have been carried out and a sizable effort has been made in laying the groundwork for such an accelerator. A second application, using ion beams for study of High Energy Density Physics (HEDP) or Warm Dense Matter (WDM) physics will soon be realized and the requirements for this machine will be discussed in detail. Also, a concept for a spallation neutron source is discussed in lesser detail.

Induction Cell Design Tradeoffs and Examples

A brief history of induction accelerator development was covered in Chap. 2. The induction accelerators constructed since the early 1960s can be categorized as short-pulse if the pulse duration is less than 100 ns and long-pulse if it is longer. The distinction between short-pulse and long-pulse is arbitrary; it mainly reflects the type of magnetic material that was typically used in the cell. Examples of short-pulse induction accelerators are the electron ring accelerator (ERA, \(\Delta t=30\) ns) [1], the advanced test accelerator (ATA, \(\Delta t=70\) ns) [2] and the experimental test accelerator (ETA-II, \(\Delta t=70\) ns) [3]. Examples of long-pulse accelerators are the Astron (\(\Delta t=400\) ns) [4, 5] and the second axis of the dual axis radiographic hydro test accelerator (DARHT-II, \(\Delta t=2{,}000\) ns) [6]. In this chapter the cell design of several of these accelerators will be described in detail. We will discuss how the physics, economics, and space requirements often lead to a non-optimum design from the accelerator systems vantage point. Although modulators are covered in Chap. 4, some specific designs will be discussed on how the constant voltage (flat-top) was achieved in concert with the cell design and compensation network .

US Heavy Ion Beam Research for High Energy Density Physics Applications and Fusion

Key scientific results from recent experiments, modeling tools, and heavy ion accelerator research are summarized that explore ways to investigate the properties of high energy density matter in heavy-ion-driven targets, in particular, strongly-coupled plasmas at 0.01 to 0.1 times solid density for studies of warm dense matter, which is a frontier area in high energy density physics. Pursuit of these near-term objectives has resulted in many innovations that will ultimately benefit heavy ion inertial fusion energy. These include: neutralized ion beam compression and focusing, which hold the promise of greatly improving the stage between the accelerator and the target chamber in a fusion power plant; and the Pulse Line Ion Accelerator (PLIA), which may lead to compact, low-cost modular linac drivers.